Oil sand, as known in the Fort McMurray region of Alberta, comprises water-wetted sand grains having viscous bitumen flecks trapped between the grains. It lends itself to separating or dispersing the bitumen from the sand grains by slurrying the as-mined oil sand in heated water so that the bitumen flecks disperse into the aqueous phase.
The bitumen in McMurray oil sand has been commercially recovered at applicant's plant for the past 25 years. Initially this was done using the following general scheme (referred to as the “hot water process”):                dry mining the oil sand at a mine site that was kilometers from an extraction plant;        conveying the as-mined oil sand on conveyor belts to the extraction plant;        feeding the oil sand into a rotating tumbler where it was mixed for a prescribed retention time with hot water (80° C.), steam, caustic and naturally entrained air to yield a slurry typically having a temperature of 80° C. During this operation, bitumen flecks were heated and became less viscous. Chunks or lumps of oil sand were ablated or disintegrated. The sand grains and bitumen flecks were dispersed or separated in the water. To some extent bitumen flecks contacted, coalesced into droplets and grew in size. The bitumen droplets contacted air bubbles and coated them or connected with them to become aerated bitumen. The term used to describe this overall process in the tumbler is “conditioning”;        the slurry produced was then diluted with additional hot water and introduced into a large, open-topped, conical-bottomed, cylindrical vessel (termed a primary separation vessel or “PSV”). The diluted slurry was retained in the PSV under quiescent conditions for a prescribed retention period. During this period, aerated bitumen droplets rose and formed a froth layer. The froth overflowed the top lip of the vessel and was conveyed away in a launder. The sand grains sank and were concentrated in the conical bottom—they left the bottom of the vessel as a wet tailings stream. Middlings, a watery mixture containing solids and relatively non-buoyant bitumen, extended between the froth and sand layers. The term used to describe this step is “spontaneous flotation”;        the tailings and middlings were withdrawn, combined and sent to a secondary flotation process carried out in a deep cone vessel wherein air was sparged into the vessel to assist with flotation. This vessel is referred to as the TOR vessel. It and the process conducted in it are disclosed in U.S. Pat. No. 4,545,892. The bitumen recovered was recycled to the PSV; and        the middlings from the deep cone vessel were further processed in air flotation cells to recover contained bitumen. The term used to describe the mechanisms in the TOR and air floatation cells are collectively referred to as “secondary flotation”.        
A fairly recent change with respect to this procedure involved:                supplying heated water at the mine site;        pre-crushing the dry, as-mined oil sand, for example to −24 inches;        mixing the pre-crushed oil sand with the heated water and entraining air, at the mine site, to form a pumpable slurry having a temperature, for example in the order of 50° C.; and        pumping the slurry through a pipeline directly to a PSV to subject it to spontaneous flotation.This procedure relies on the mechanisms of conditioning being completed satisfactorily as the slurry moves through the pipeline so that, when the slurry is retained in the PSV, a viable proportion of the contained aerated bitumen reports to the froth layer.        
It needs to be understood that the composition and processability of oil sand varies, often significantly. As a consequence, we have noted that, from time to time, the pipelined oil sand slurry produces unsatisfactory primary froth yield in the PSV. We believe that one reason for this may be that the residence time in the pipeline may be too short for that particular oil sand and that the bitumen droplets do not coalesce and grow to a size suitable for aeration and flotation.